This invention relates in general to a casting machine and in particular to an improved method and apparatus of venting a fluid from a lined pressure furnace of such a casting machine.
Pressure pouring of molten metal from a furnace to fill a mold cavity has been used for several decades despite a number of problems. At room temperature, the metal is solid and becomes fluid when melted with sufficient heat. It is known to use a low pressure countergravity casting apparatus to cast molten metal into a mold. One example of such an apparatus is described in U.S. Pat. No. 5,215,141. Basically, in a low pressure countergravity casting apparatus, molten metal is supplied to a machine furnace under pressure. The molten metal is first received into a furnace of the machine furnace. The molten metal in the furnace is then transported to a mold though a feed tube. The machine furnace includes a supply conduit for introducing a gas under pressure into the machine furnace. As the gas is introduced into the machine furnace, the molten metal in the machine furnace is forced through a submerged feed tube, or evacuation conduit, into the mold. The evacuation conduit is commonly referred to as a stalk tube. The mold receives the molten metal through holes in the bottom of the mold. The molten metal in the mold cooling and hardening produces a cast article. A controller is used to adjust the pressure at which the gas is being introduced into the machine furnace. Thus, it can be seen that the machine furnace, the casting apparatus, and the mold are in fluid communication.
One potential problem during a casting operation is that the lined pressure furnace can suffer from excess gases flowing up though a bath of molten metal when the furnace is depressurized. In normal pressure casting operations, gases are introduced into the porous lining of the furnace during the pressurization cycle, which often lasts more than sixty seconds. Upon rapid depressurization, gases that have infiltrated the porous lining of the furnace seek the easiest way or path of least resistance out of the lining. Much of the trapped gas finds its way out of the porous lining below the surface of the melt and can contaminate the melt with oxygen as the gas rises to the surface (as shown by the phantom arrows X3 in FIG. 3). As the gas rises in the melt, the gas tends to effervesce or bubble thereby creating a greater interaction with, and thus contamination of, the melt. The porous lining as well as the riser tubes can also become contaminated with oxide formation. The rising gases can also stir sediment from the bottom of the furnace thereby adding contamination to the melt. When immersion heaters are used in this type of furnace, the gas bubbles that come into contact with the surface of the heaters form insulating oxides on the surface of the heaters, thereby reducing their effectiveness. Bubbles that are small rise slowly and some are in suspension near the riser tubes long enough to be forced into the next casting when the next cycle begins, thereby degrading the quality of the castings. Thus, it would be desirable to provide an improved method and apparatus for a pressure lined furnace of a casting machine which is operative to reduce the contamination of the melt by providing an easier path for the gases to escape from the lining of the furnace.